While Blizzard Entertainment's 2010 World of Warcraft expansion pack Cataclysm wasn't the first expansion for the massively popular MMORPG, it was certainly the most ambitious.

At GDC 2011 on Wednesday, World of Warcraft game director Tom Chilton said updating the game through Cataclysm proved to be a difficult balancing act between maintaining the good aspects of the original and adding new and improved features.

"The 'world' part of World of Warcraft is arguably its strongest asset," said Chilton - that had to be preserved, even though Cataclysm was a massive overhaul for the MMO that reflected a cataclysmic event that affected the entire world of Azeroth.

Chilton and the team at Blizzard had to act as plastic surgeons, extracting the fat and lifting the sagging parts while retaining the underlying beauty that has attracted 12 million gamers to the MMO.

"We really knew we had to retain the soul of the original," said Chilton. "...This is where we really knew we could fuck it up." The game's world was inviting, but "the content and the mechanics were showing their age," said Chilton.

One of the zones in World of Warcraft that Blizzard was to update for Cataclysm was Desolace. Chilton described the zone as monotonous, oppressive and, as the name implies, desolate. He also said that the zone had very poor hubbing, flow and travel. "If you wanted to do any quest, you pretty much have to walk across the whole zone," he said.

With the expansion, Blizzard updated the visuals, story progression and updated the mechanics, flow and travel. But Chilton questioned whether that was the right decision.

"I feel like this is a case that it lost a little bit of its soul. ... I think we really have to watch where we take from the original. Really keep in mind what the soul of the original was. Make sure it's a really conscious decision and you're doing it for the right reason."

There was also the case of updating the zone Westfall, a memorable zone in World of Warcraft that had some of the oldest mechanics in the game, being about the second zone that Blizzard had created.

For that zone, Blizzard retained the overall look, made minor cataclysmic terrain changes, improved quest mechanics and progressed the story line. And Chilton was happy with those small changes, which let the zone keep its soul. "very subtle changes can have a big impact ... and not compromise the feel," he said.

Not only did Blizzard update the game itself, but its production strategy had matured by the time Cataclysm was in development. When the studio makes quests today, the team creates visual flow charts and lays out where the story is going to be in the zone, the cutscenes and the quests. That visualization makes it easier to make changes and improvements, and other departments can more easily see how the zone will work.

And when updating a game or releasing a sequel, a common problem is that features continue to stack up to overwhelming levels. In the case of World of Warcraft, Chilton used the example of the game's ever-expanding talent tree, which had grown to become quite complicated.

"You start to realize that this is getting really overwhelming, and we decided that with Cataclysm it was time for it to stop," he said. "...We ended up with talent trees that were roughly the same size as the version 1.0 talent trees," which were much simpler and smaller.

Some players complained that the decision limited customization for characters. But Chilton argued, "I assure you that there are not fewer viable builds today than when we shipped [previous expansion] Lich King."

"If you were a returning or player, [the huge talent tree] would be really intimidating," he said. "A smaller amount of choices can help them make better choices."

As a general rule, Chilton suggested the rule of thirds for an update or sequel to a game: 1/3 old, 1/3 improved and 1/3 new. In all, Chilton advised not to forget what made your game great in the first place. "Know what was great about awesome game one, and keep that for awesome game two," he said.